In the fabrication of large steel structural members such as portal frames, roof trusses and structural beams, various elements including webs and flanges are secured by manual or automated welding processes. Typically, the web and flange elements are arranged in a predetermined juxtaposition on a surface or a purpose built support jig to facilitate joining of the various elements, usually by a fillet welding process.
Initially, the web may be supported horizontally with the flange elements positioned in an upright manner against the edges of the web. In some cases, reinforcing braces or flange support members extend across the face surface of the web between adjacent flanges thus presenting both horizontal and vertical weld tracks. Because of the size of the weld bead(s) to be deposited manually or otherwise due to limitations in automated welding apparatus, most weld beads are laid in a horizontal position.
Accordingly, in the fabrication of a structural beam with a web, main flanges, web reinforcing ribs, mounting brackets and the like, it is necessary to rotate the beam at least through four 90° quadrants during the fabrication process in order to present horizontally oriented weld bead positions. Typically, rotation of the beam during the fabrication process is effected by a gantry crane supporting the beam while balanced in a loose sling such that as the beam is elevated above the jig or support base, it is able to rotate under the influence of gravity through at least 45° whereby when lowered by the crane back onto the jig or support base, it completes a 90° quadrant rotation to present a series of horizontal weld bead positions on each side of the now upright web. This process is repeated three times to effect welding of all joints with horizontally laid weld beads.
While generally effective for its intended purpose, the use of a gantry crane is quite inefficient in terms of labour intensity and the need for fabrication jigs to be located longitudinally under the travel shadow of the gantry crane.
Moreover, the requirement to balance such beams in a support sling during rotation about a longitudinal axis is a quite dangerous procedure, particularly for tapered beams necessitating a non-central location of the supporting sling. A further disadvantage of this handling method is that a separate sling is required for differing beam sizes and configurations.
Various mechanisms for rotation of large objects about a horizontal axis are known. For example, U.S. Pat. Nos. 4,053,365, 5,437,707 and 6,860,735 all describe rotary kilns comprising an inclined cylinder supported for rotation by idler rollers engaging a circumferential track. The cylindrical kilns may be driven by a toothed pinion engaging in a toothed circumferential rack extending about the cylinder, a chain and sprocket drive, etc. While generally effective for their intended purpose, they require special thrust bearings to support the load of the inclined cylinder and otherwise are only suited to driving elongate cylindrical members.
An alternative mechanism for rotation of large objects about a longitudinal axis comprises a powered sling mechanism supportable from a gantry crane hook. The sling may be an endless loop of chain or cable and is powered by a drive motor remotely actuable by an operator. Again, this system does not really avoid the shortcomings and safety issues associated with the rotation about a longitudinal axis of large objects such as structural members whilst suspended by a gantry crane or the like.
Accordingly, it is an object of the invention to overcome or alleviate at least some of the disadvantages associated with prior art methods and apparatus for rotating large structural members about a longitudinal axis and otherwise provide steel fabricators and the like with a convenient choice.